Alignment pattern detecting sensor, method of determining acceptance width of the alignment pattern detecting sensor, method of forming alignment pattern, and image forming apparatus

ABSTRACT

An image forming apparatus includes an alignment pattern and an alignment pattern detector that detects the alignment pattern. The alignment pattern is formed in such a manner that a line image of a reference color and a line image of a color other than the reference color are superposed on each other with a predetermined shift amount. An acceptance width of the alignment pattern detector is determined so that the acceptance width satisfies a relation with a writing density of the image forming apparatus and a line width of the alignment pattern, as follows:
 
[acceptance width]&gt;[line width]/(5.0627×[writing density(dpi)] −0.5331 ).

CROSS-REFERENCE TO RELATED APPLICATIONS

This document is a Continuation Application of and is based upon andclaims the benefit of priority under 35 U.S.C. §120 from U.S. Ser. No.10/722,490, filed Nov. 28, 2003. The present document also incorporatesby reference the entire contents of Japanese priority document,2002-347814 filed in Japan on Nov. 29, 2002 and 2002-347844 filed inJapan on Nov. 29, 2002.

BACKGROUND OF THE INVENTION

1) Field of the Invention

The present invention relates to an image forming apparatus that formscolor images, an alignment pattern detecting sensor for the imageforming apparatus, a method of determining an acceptance width for thealignment pattern detecting sensor, and a method of forming an alignmentpattern.

2) Description of the Related Art

Conventionally, a color image forming apparatus that has aphotosensitive drum and a revolver type developing device takes a mainstream in the market. This type of color image forming apparatus formstoner images of respective colors, superposes the toner images on anintermediate transfer element to make a combined color image, formstoner images of respective colors, superposes the toner images on anintermediate transfer element to make a combined color image, andbatch-transfers the combined color image to a recording medium.

On the other hand, with a recent demand of high speed and highperformance for the color image forming apparatus, a four-drum tandemtype color image forming apparatus has become popular. The tandem typecolor image forming apparatus has a configuration that a plurality ofimage forming units each including a pair of a photosensitive element(image carrier) and a developing device are disposed for each color atpositions near a transfer belt, and toner images on the image carriersare sequentially transferred to a recording medium or a transfer belt toform the combined color image.

With this type of color image forming apparatus, since the toner imagesformed on the image carriers for respective colors can be transferredvirtually at the same time, the tandem type color image formingapparatus has an advantage in that the printing speed can be increased.On the other hand, there is a disadvantage with respect to colormisalignment between the respective colors, as compared to theconventional one-drum intermediate transfer type color image formingapparatus.

With regard to the technological problem of the color misalignment, manycorrection methods have been proposed. For example, Japanese PatentKokoku No. Hei 7-19084 discloses a technology of correcting the colormisalignment by forming line images for respective colors on a transferbelt, detecting passing of the line images by a detecting sensor, andmeasuring each offset from an ideal passing timing of the line images.

Since such a technology is a method of detecting an edge of a patternpassing through the detecting sensor, the detection accuracy isdetermined by the sampling frequency. In other words, if a machine has aresolution of 600 dpi and the correction unit is 42.3 micrometers(=25.4/600×1000), at least a detection of ±½ of the correction unit(=21.7 micrometers) is required. When the linear velocity of the lineimage on the transfer belt is 125 mm/sec, a minimum required samplingfrequency is calculated as at least 6 kilohertz, from an equation of[sampling frequency]=[linear velocity]/[25.4/resolution dpi/2], but thedetection accuracy (=detection error) in this case (=6 kHz) becomes 21.7micrometers.

If this calculation is directly fed back to the misalignment correction,there may be no problem with such a degree of sampling frequency.However, there is a case where it is necessary to use this detectionresult (=x micrometers) for other operation. For example, when suchdetection is performed at left and right opposite ends of a recordingmedium with respect to the recording medium conveying direction and skewcorrection is performed or magnification error correction is performed,based on the detection results at the opposite ends, higher detectionaccuracy is required. Therefore, for example, when 2 micrometers arerequired as the detection accuracy, it is necessary to increase thesampling frequency to as high as 60 kilohertz.

Since the necessary sampling frequency is in proportion to the linearvelocity and resolution, high processing speed capable of coping withthe high-speed sampling becomes necessary for processing steps after thedata sampling. Consequently, a cost required for color misalignmentcorrection increases in substantial proportion to the increase in thespeed of apparatus.

As detecting means for improvement in detection accuracy of patternedge, a method of detecting the pattern edge by a charge coupled device(CCD) sensor having high accuracy and high resolution is proposed.However, even if such detecting means is used, problems such ascomplication and cost increase of the apparatus cannot be avoided.

For example, Japanese Patent No. 3254244 discloses the technology asfollows. A toner image pattern is formed by superposing a second colortoner image on a first color toner image and other toner image patternsare formed by shifting the relative position of the two color patternsby a predetermined amount. Each average density of the toner imagepatterns is detected by an optical sensor, and a misalignment betweenthe first color and the second color and the direction of themisalignment are determined from output signals of the optical sensor tocorrect the misalignment.

In this technology, the detection of the misalignment is performed notby detecting the edge of a pattern image (line image), but by detectingan average output signal of the optical sensor based on the wholepattern. Therefore, it is possible to detect the misalignment at asampling frequency as low as 500 hertz or below (for every 2milliseconds), that is, about 1/100 times low as compared with thetechnology disclosed in Japanese Patent Kokoku No. Hei 7-19084.

Therefore, if a detection accuracy of the same level as that of thetechnology disclosed in Japanese Patent Kokoku No. Hei 7-19084 can beobtained by using the misalignment detection method disclosed inJapanese Patent No. 3254244, the hardware can be configured at a lowercost relating to the detection of misalignment, and therefore,considerable cost reduction can be obtained.

The technology similar to the misalignment detection method described inJapanese Patent No. 3254244 includes technologies disclosed, forexample, in Japanese Patent Application Laid-Open No. Hei 10-329381,Japanese Patent Application Laid-Open No. 2000-81745, Japanese PatentApplication Laid-Open No. 2001-209223, Japanese PatentApplicationLaid-Open No. 2002-40746, and Japanese Patent Application Laid-Open No.2002-229280.

With regard to the technology of misalignment correction disclosed inJapanese Patent No. 3254244, if the maximum correction amount that hasto be corrected is ±10 dots, then the misalignment correction amount andthe direction thereof can be determined by forming 21 patterns obtainedthrough shifting the relative position of the two colors dot by dot andreading extreme values of the patterns.

However, creating that many patterns increases not only wasteful tonerconsumption, but also the time required for automatic misalignmentadjustment, which is not desirable.

Japanese Patent Application Laid-Open No. Hei 10-329381 discloses amethod of detecting a misalignment more accurately, by calculating anintersection point of two lines when a reflected optical density isplotted on the y-axis with respect to a printing position parameterplotted on the x-axis.

In the method disclosed in Japanese Patent Application Laid-Open No. Hei10-329381, even if the maximum correction amount is ±10 dots, it is notnecessary to form 21 patterns, and only 11 patterns may be formedthrough shifting by several dots appropriately, for example, by 2 dots.If the pattern is shifted by 5 dots, then only five patterns arerequired. Thus, highly accurate misalignment correction can be realized,while considerably reducing the number of patterns and the time requiredfor misalignment adjustment.

Since misalignment adjustment is an operation that has nothing to dowith the actual printing operation, if the processing time is too long,the time required for the first print increases accordingly. Therefore,the shorter time for the adjustment is better, considering theproductivity.

However, there is a case where a positional deviation is obtained bycalculating an intersection point of a linear approximate expression oftwo lines, and there is also a case where a deviation is obtainedthrough arithmetic operation with a resolution having a shift amount ofthe line or less. In either of the cases, such an output characteristicas follows must be obtained. Specifically, a sensor output signal ofeach patch linearly increases or decreases with respect to apredetermined shift amount, that is, a line in which a determinationcoefficient R² of each approximate expression of two lines is infinitelyclose to 1, must be obtained.

Therefore, by using a one-drum intermediate transfer belt type colorimage forming apparatus (writing density: 600 dpi) as illustrated inFIG. 33, it is verified whether the same degree of detection accuracy asthat of the edge detection method disclosed in Japanese Patent KokokuNo. Hei 7-19084 is obtained through density detection of a two-colorsuperposed pattern and calculation of the intersection point. Asillustrated in FIG. 37, a patch is formed by superposing two color linesof black (Bk) as a reference color and another color (for example, cyan(C)), and a patch is also formed as one line as the minimum number ofthe lines obtained by superposing the two color lines. A detectionpattern (alignment pattern) Pk for misalignment detection in the mainscanning direction is obtained by continuously forming 13 patches (P1 toP13) with the relative position of the two colors shifted by anarbitrary amount. This detection pattern is read by a conventionaloptical sensor (alignment pattern detecting sensor) as illustrated inFIG. 38A, and output voltages of each of the patches with respect to theshift amount of the line other than the reference color are plotted tocalculate an intersection point. Experiments were conducted on tworeferences of 24 dots and 10 dots as references for line widths of twocolors. The reason why the one-drum intermediate transfer type imageforming apparatus was used is because it is desirable to keep aninfluence of the apparatus from a verified result as low as possible.The pattern used for verification was used for the pattern in the mainscanning direction for the same reason as explained above.

As illustrated in FIG. 37, the respective patches are arranged along thescanning direction of the optical sensor, i.e., the direction of thetransfer belt movement, and the color other than the reference color isshifted by an arbitrary amount in a direction orthogonal to thedirection of the movement, in order to detect color misalignment in themain scanning direction.

As illustrated in FIG. 38A and FIG. 38B, the optical sensor includes alight emitting diode (LED) 700, a regular reflected light receivingelement 701, and a diffused light receiving element 702, and theseelements are supported by a support base 703. These elements areactually arranged in a substantially vertical plane with respect to amoving plane of an alignment pattern, but FIG. 38A illustrates thearrangement as a plan view obtained by rotating it by 90 degrees forsimplicity. As illustrated in FIG. 38B, the support base 703 has a spotshape 700 a of the LED 700, a spot shape 701 a of the regular reflectedlight receiving element 701, and a spot shape 702 a of the diffusedlight receiving element 702.

FIG. 39 illustrates the result of a case where the line width is 24dots, and FIG. 40 illustrates the result of a case where the line widthis 10 dots.

As illustrated in FIG. 39, in the case of the line width: 24 dots, inthe approximate line obtained by plotted points on the negative sidewith respect to the extreme value, R² is 0.9275. On the other hand, inthe approximate line obtained by plotted points on the positive sidewith respect to the extreme value, R² is 0.9555. Thus, the obtainedoutput characteristic is not a straight line at all.

As a result of calculation of an intersection point based on the twoapproximate lines, a positional deviation of 34.74 micrometers (=0.82dot) is obtained.

On the other hand, as illustrated in FIG. 40, in the case of the linewidth: 10 dots, in the approximate line obtained by plotted points onthe negative side with respect to the extreme value, R² is 0.9909. Onthe other hand, in the approximate line obtained by plotted points onthe positive side with respect to the extreme value, R² is 0.9985. Thus,the output characteristic quite close to a straight line is obtained.

As a result of calculation of an intersection point by the twoapproximate lines, a positional deviation of 12.91 micrometers (=0.30dots) is obtained. The experiment conditions are as follows.

Detailed Parameters of the Detection Pattern as illustrated in FIG. 39:

-   -   Line width: 24 dots (=25.4/600×1000×24=1.016 millimeters) . . .        commonly set for the Bk line and the color line    -   Shift amount: 4 dots (=25.4/600×1000×4=169.3 micrometers)    -   Total number of patches: 13 patches (at P1 and P13, the two        lines are not superposed perfectly, but at P7, the two lines are        perfectly superposed on each other)    -   Repetition number of line: 1        Detailed Parameters of the Detection Pattern as illustrated in        FIG. 40:    -   Line width: 10 dots (=25.4/600×1000×10=0.423 millimeters) . . .        commonly set for the Bk line and the color line    -   Shift amount: 1 dot (=25.4/600×1000×1=42.3 micrometers)    -   Total number of patches: 21 patches (at P1 and P21, the two        lines are not superposed perfectly, but at P11, the two lines        are perfectly superposed on each other)    -   Repetition number of line: 1        Detecting Sensor (detailed specification of the sensor as        illustrated in FIG. 38A and FIG. 38B):        Light Emission Side:    -   Element: GaAs infrared light emitting diode (peak emission        wavelength, λ_(p)=950 nanometers)    -   Top view type spot diameter: 1.0 millimeter        Light Reception Side:    -   Element: Si phototransistor (peak spectral sensitivity,        λ_(p)=800 nanometers)    -   Top view type spot diameter:        -   Regular reflected light receiving side: 1.0 millimeter        -   Diffused light receiving side: 3.0 millimeters    -   Detection distance: 5 millimeters (distance from upper part of        the sensor to a target surface (patch) to be detected)        Linear Velocity:    -   245 mm/sec [sampling frequency]    -   500 sampling/sec

In the experiments, the alignment pattern was formed on the transferbelt so as to substantially match between the center of the Bk line andthe center of the light receiving plane of the sensor.

As explained above, it is confirmed that the linearity as a basis ofcalculation of the intersection point is changed due to different linewidths. This means that the detection accuracy of misalignment can befurther improved by using a more appropriate method of forming thealignment pattern. In other words, this means that improvement in thedetection accuracy of misalignment is achieved without cost increase.

SUMMARY OF THE INVENTION

It is an object of the present invention to solve at least the problemsin the conventional technology.

The method of determining an acceptance width for an alignment patterndetector according to one aspect of the present invention includesderiving a correlation between a line width of the alignment pattern, awriting density of the image forming apparatus, and the acceptance widthof the alignment pattern detector, and determining the acceptance widthbased on the correlation derived.

The method of forming an alignment pattern for an image formingapparatus according to another aspect of the present invention includesderiving a correlation between a line width of the alignment pattern, awriting density of the image forming apparatus, and an acceptance widthof the alignment pattern detector, determining the line width based onthe correlation derived, and forming the alignment pattern on a mediumbased on the line width determined.

The alignment pattern detecting sensor according to still another aspectof the present invention detects an alignment pattern on a medium in animage forming apparatus. The alignment pattern is formed on a medium bysuperposing a line image of a reference color and a line image of asample color other than the reference color, and an acceptance width ofthe alignment pattern detecting sensor is determined from followinginequality[acceptance width]>[line width]/(α×[writing density (dpi)]^(−β)).

The image forming apparatus according to still another aspect of thepresent invention includes an alignment pattern forming unit that formsan alignment pattern on a medium by superposing a line image of areference color and a line image of a sample color other than thereference color, an alignment pattern detector that detects thealignment pattern, and a misalignment correcting unit that, based onoutput signals of the alignment pattern detector, determines an amountand a direction of a misalignment between the line images of the twocolors, and corrects the misalignment. The acceptance width of thealignment pattern detector, a line width of the alignment pattern, and awriting density of the image forming apparatus satisfy followinginequality[acceptance width]>[line width]/(α×[writing density (dpi)]^(−β)).

The computer program for determining an acceptance width for analignment pattern detector, which detects an alignment pattern in animage forming apparatus, according to still another aspect of thepresent invention makes a computer execute the method of determining anacceptance width for an alignment pattern detector according to thepresent invention.

The other objects, features and advantages of the present invention arespecifically set forth in or will become apparent from the followingdetailed descriptions of the invention when read in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a color printer as an image formingapparatus according to the first embodiment of the present invention;

FIG. 2 is a schematic diagram of an alignment pattern for misalignmentdetection in the main scanning direction;

FIG. 3A and FIG. 3B are schematic diagrams for explaining an arrangementof the alignment pattern detecting sensor with respect to the scanningdirection of the alignment pattern;

FIG. 4 is a schematic diagram for explaining a positional relationbetween the alignment pattern on the transfer belt and the alignmentpattern detecting sensor;

FIG. 5 is a block diagram of a misalignment correcting unit;

FIG. 6 is a graph of actually measured values of the color line width ineach patch with respect to the Bk line using a microscope;

FIG. 7A to FIG. 7C are schematic diagrams for explaining a non-linearityin an increase in each area caused by a circular light receiving planeof the alignment pattern detecting sensor (with 24-dot line width);

FIG. 8A to FIG. 8C are schematic diagrams for explaining a non-linearityin an increase in each area caused by a circular light receiving planeof the alignment pattern detecting sensor (with 10-dot line width);

FIG. 9 is a schematic diagram for explaining how to obtain the area of arectangle in the circular light receiving plane;

FIG. 10A is a graph of output values of patches obtained by performingsimulation of the area of a color line image occupied in the lightreceiving plane with a condition shown in FIG. 10B;

FIG. 10B is a schematic diagram of a positional relation between thecenter of the Bk line and the center of the light receiving plane (withboth centers matched);

FIG. 11A is a graph of output values of patches obtained by performingsimulation of the area of a color line image occupied in the lightreceiving plane with a condition shown in FIG. 11B;

FIG. 11B is a schematic diagram of a positional relation between thecenter of the Bk line and the center of the light receiving plane (withboth centers mismatched by 12 dots);

FIG. 12A is a graph of output values of patches obtained by performingsimulation of the area of a color line image occupied in the lightreceiving plane (with a condition shown in FIG. 12B);

FIG. 12B is a schematic diagram of a positional relation between thecenter of the Bk line and the center of the light receiving plane (withboth centers mismatched by 36 dots);

FIG. 13 is a graph of a detection error with respect to a deviation ofthe center of the Bk line from the center of the light receiving plane;

FIG. 14 is a graph of a detection error with respect to a linewidth-to-light receiving diameter ratio;

FIG. 15 is a graph of a relation between resolution and a maximumtolerance of the line width-to-acceptance width ratio;

FIG. 16 is a graph of a relation between a shift amount of the colorline with respect to the Bk line and an output voltage of diffusedlight, illustrating the state in which 12 dots are deviated;

FIG. 17 is a graph of a relation between a shift amount of the colorline with respect to the Bk line and an output voltage of diffusedlight, illustrating the state in which 24 dots are deviated;

FIG. 18 is a graph of the result of measuring a deviation of thealignment pattern on the transfer belt;

FIG. 19 is a graph of the result of measuring a deviation of thealignment pattern on the transfer belt by the sensor;

FIG. 20A and FIG. 20B are schematic diagrams for explaining a state inwhich line thickening occurs in both the Bk line and the C line;

FIG. 21A and FIG. 21B are schematic diagrams for explaining a state inwhich line thickening occurs only in the Bk line;

FIG. 22 is a graph of a relation between LED light emission current andoutput of regular reflected light;

FIG. 23 is a graph of the output of the misalignment pattern whendetection is performed by regular reflected light;

FIG. 24 is a graph of a relation between LED light emission current andoutput of diffused light;

FIG. 25 is a graph of the output of the misalignment pattern, whendetection is performed by diffused;

FIG. 26 is a graph of a relation between glossiness on the surface ofthe transfer belt and the sensor output;

FIG. 27 is a graph of a relation between lightness on the surface of thetransfer belt and the output of diffused light;

FIG. 28 is a graph of a relation between the shift amount of the colorline with respect to the Bk line and an output voltage of diffusedlight;

FIG. 29 is a schematic diagram of the alignment pattern for misalignmentdetection in the sub-scanning direction;

FIG. 30A to FIG. 30C are schematic diagrams for explaining changes ineach orientation (angle) of the alignment pattern with respect to thescanning direction;

FIG. 31 schematic diagram of an alignment pattern detecting sensoraccording to the second embodiment of the present invention;

FIG. 32 is a schematic diagram of a color copying machine as an imageforming apparatus according to the third embodiment of the presentinvention;

FIG. 33 is a schematic diagram of a color copying machine as an imageforming apparatus according to the fourth embodiment of the presentinvention;

FIG. 34 is a schematic diagram of an ink jet printer as an image formingapparatus according to the fifth embodiment of the present invention;

FIG. 35 is an enlarged diagram of a conveyor belt and nearby parts inFIG. 34;

FIG. 36 is a schematic diagram of the positional relation of thealignment pattern and the alignment pattern detecting sensor in the inkjet printer;

FIG. 37 is a schematic diagram of an alignment pattern formed bysuperposing two color lines;

FIGS. 38A and 38B are schematic diagrams for explaining an arrangementof respective elements in the conventional optical sensor, with respectto the scanning direction of the alignment pattern;

FIG. 39 is a graph of the output voltages of the misalignment patternwith a line width of 24 dots and a calculation result of theintersection point, when the conventional optical sensor is used; and

FIG. 40 is a graph of the output voltages of the misalignment patternwith a line width of 10 dots and a calculation result of theintersection point, when the conventional optical sensor is used.

DETAILED DESCRIPTION

Exemplary embodiments of an alignment pattern detecting sensor, a methodof determining acceptance width of the alignment pattern detectingsensor, a method of forming alignment, and an image forming apparatusaccording to the present invention is explained in detail with referenceto the accompanying drawings

FIG. 1 is a schematic diagram of a color printer as an image formingapparatus according to the first embodiment of the present invention.The color printer has three paper feed trays, one manual feed tray 36,and two paper feed cassettes 34 (first paper feed tray) and 34 (secondpaper feed tray). Recording medium (not shown) as a sheet-type recordingmedium fed by the manual feed tray 36 is separated one by onesequentially from the uppermost sheet by a feed roller 37, and conveyedtoward a registration roller pair 23. Recording medium fed from thefirst paper feed tray 34 or the second paper feed tray 34 is separatedone by one sequentially from the uppermost sheet by a feed roller 35,and conveyed toward the registration roller pair 23 via a conveyingroller pair 39.

The fed recording medium is once stopped by the registration roller pair23, and skew is corrected. Thereafter, the recording medium is conveyedtoward a transfer belt 18 by a rotation operation of the registrationroller pair 23 by ON control of a registration clutch (not shown), at atiming at which the front end of an image formed on a photosensitivedrum 14Y, explained later, located on the uppermost stream and apredetermined position of the recording medium in its conveyingdirection coincide with each other.

When the recording medium passes through a nip for attracting a paper,the nip being formed with the transfer belt 18 and a paper attractionroller 41 in contact with the belt 18, the recording medium iselectrostatically attracted to the transfer belt 18 by a bias applied tothe paper attraction roller 41, and conveyed at a process linearvelocity of 125 mm/sec.

Transfer bias (positive) having a polarity opposite to the chargedpolarity (negative) of the toner is applied to transfer brushes 21B,21C, 21M, and 21Y arranged at positions facing the photosensitive drums14B, 14C, 14M, and 14Y of the respective colors, with the transfer belt18 put between the brushes and drums. Thereby, toner images in therespective colors formed on the photosensitive drums 14B, 14C, 14M, and14Y are transferred to the recording medium attracted to the transferbelt 18, in order of yellow (Y), magenta (M), cyan (C), and black (Bk).

The recording medium having passed through transfer steps for the colorsis separated from the transfer belt 18 by the curvature of a driveroller 19 provided on the downstream side, and conveyed to a fixingdevice 24. By passing through a nip for fixing the image, the nip beingformed with a fixing belt 25 and a pushing roller 26 in the fixingdevice 24, the toner image is transferred to the recording medium byheat and pressure. The recording medium with the image fixed thereon isejected to an image face-down (FD) tray 30 formed on the upper surfaceof the body of the apparatus, in a single-sided printing mode.

When a double-sided printing mode is selected in advance, the recordingmedium output from the fixing device 24 is conveyed to a reversing unit(not shown), the recording medium is turned upside down therein, andconveyed to a double-sided conveying unit 33 located below the transferunit. The recording medium is re-fed from the double-sided conveyingunit 33, and conveyed to the registration roller pair 23 via theconveying roller pair 39. Thereafter, through the similar operation tothat in the single-sided printing mode, the recording medium passes thefixing device 24 and is ejected to the FD tray 30.

The image forming section is provided in plurality for the respectivecolors, and the image forming sections have the same configuration andoperation as each other. Therefore, only the configuration and operationfor forming a yellow image are representatively explained herein.

An image forming unit 12Y that has a charging roller 42Y and a cleaningunit 43Y, a developing unit 13Y, and an optical write unit 16 areprovided around the photosensitive drum 14Y located on the most upstreamside in the direction of conveying the recording medium.

At the time of image forming, the photosensitive drum 14Y is rotated inthe clockwise direction by a main motor (not shown), and decharged byalternate current (AC) bias (having no direct current (DC) component)applied to the charging roller 42Y, and thereby, the surface potentialbecomes a reference potential of about −50 volts.

The photosensitive drum 14Y is uniformly charged to a potentialsubstantially equal to the DC component by applying the DC bias on whichthe AC bias is superposed, to the charging roller 42Y, and the surfacepotential thereof is charged substantially to −500 volts to −700 volts(the target charged potential is determined by a process controller).

The digital image information as a print image sent from a controller(not shown) is converted to a binary LD light emission signal for eachcolor, and exposure light 16Y is radiated onto the photosensitive drum14Y by the optical write unit 16 including a cylinder lens, a polygonmotor, an f/θ lens, first to third mirrors, a WTL lens, and the like.

The potential on the light radiated surface of the drum becomessubstantially −50 volts, and an electrostatic latent image correspondingto the image information is formed at a writing density (=resolution) of600 dpi.

The electrostatic latent image corresponding to the yellow imageinformation on the photosensitive drum 14Y is visualized by thedeveloping unit 13Y. By applying DC (−300 to −500 v) on which AC bias issuperposed to a developing sleeve 44Y in the developing unit 13Y, onlythe image portion where the potential has dropped due to writing isdeveloped with toner (Q/M, −20 to −30 μC/g), and a toner image isformed. The developing unit 13Y is a developing device using a so-calledtwo-component developer, in which a mixed developer of carrier and tonerare contained.

The toner image formed on each of the photosensitive drums 14B, 14C,14M, and 14Y for respective colors is transferred to the recordingmedium attracted to the transfer belt 18, by the transfer bias.

In the color printer of the embodiment, prior to such image formingoperation, color misalignment is adjusted. The color misalignmentadjusting operation is performed by forming an alignment patternexplained later on the transfer belt 18, and reading (detecting) thealignment pattern by the alignment pattern detecting sensor(hereinafter, “sensor”) 40 as an alignment pattern detector.

The alignment pattern detecting sensor 40 is arranged on the lower sideof the transfer belt 18, facing the photosensitive drum 14B.

An alignment pattern Pm for detecting a misalignment in the mainscanning direction is obtained as illustrated in FIG. 2, by designatinga plurality of lines, as one patch, formed by superposing the line imageBk of black as the reference color and a line image of a color otherthan the reference color such as a line image C of cyan, andcontinuously forming patches by shifting the relative position of theline images of the two colors by an arbitrary amount (by an arbitraryshift amount). However, it does not mean that the reference color isrestricted to black (the same applies in the other embodiments asfollows).

Here, “arbitrary shift amount” includes a case when the shift amount isnot always constant, such that a shift amount between P1 and P2 is 50micrometers, and a shift amount between P2 and P3 is 20 micrometers.

In this embodiment, the alignment pattern Pm has a configuration suchthat one patch is formed by superposing a black line having a width of250 micrometers on a color line other than the reference color havingthe same width, and the whole pattern is formed with 13 pieces of suchpatches. In the respective patches, each color line C is shifted by 40micrometers with respect to each Bk line.

Here, “continuously forming patches” means that the patches are arrangedalong the scanning direction (in the advance direction of the transferbelt 18). Therefore, even if the patches are arranged on a random basis,such as in order of P1, P11, P2, and P10, this case is also included in“continuously forming”. Even if the interval between P1 and P2 is notthe same as the interval between P2 and P3, this case is also includedin “continuously forming”.

The alignment pattern detecting sensor 40 in the embodiment has the sameconfiguration as that of the conventional sensor as illustrated in FIG.38A and FIG. 38B. In other words, as illustrated in FIG. 3A and FIG. 3B,the detecting sensor 40 includes a light emitting diode (LED) 40A as alight emitter, a regular reflected light receiving element(phototransistor) 40B as a light receiver, and a diffused lightreceiving element (phototransistor) 40C (not shown) as a light receiver.These elements are supported by the support base 40D (not shown).Reference numeral 40A-1 represents a spot shape of the LED 40A, 40B-1represents a spot shape of the regular reflected light receiving element40B, and 40C-1 (not shown) is a spot shape of the diffused lightreceiving element 40C (not shown). The specifications of the alignmentpattern detecting sensor 40 are as follows. Detecting Sensor (detailedspecification of the sensor as illustrated in FIG. 3A and FIG. 3B):

Light Emission Side:

-   -   Element: GaAs infrared light emitting diode (peak emission        wavelength, λ_(p)=950 nanometers)    -   Top view type spot diameter: 1.0 millimeter        Light Reception Side:    -   Element: Si phototransistor (peak spectral sensitivity,        λ_(p)=800 nanometers)    -   Top view type spot diameter:        -   Regular reflected light receiving side: 1.0 millimeter        -   Diffused light receiving side: 3.0 millimeters    -   Detection distance: 5 millimeters (distance from upper part of        the sensor to a target surface (patch) to be detected)

The alignment patterns Pm are formed at three positions, opposite endsand the center of the transfer belt 18 as illustrated in FIG. 4.Corresponding three pieces (40 a, 40 b, 40 c) of the alignment patterndetecting sensor 40 are provided by being supported by support bases(not shown). The two sensors provided at the opposite ends of thetransfer belt 18 detect a misalignment of the alignment pattern Pm fordetecting a misalignment in the main scanning direction as illustratedin FIG. 2 to thereby correct the misalignment in the main scanningdirection and a magnification error. The three sensors provided at theopposite ends and the center of the transfer belt 18 detect amisalignment of an alignment pattern Ps for detecting a misalignment inthe sub-scanning direction as illustrated in FIG. 29 to thereby correctthe misalignment in the sub-scanning and skew.

The color printer as the image forming apparatus of the embodiment has awriting density of 600 dpi. Accordingly, a line width of each patch inthe alignment pattern Pm and an acceptance width of the alignmentpattern detecting sensor 40 are set so as to satisfy relations betweenthem as follows.[line width/acceptance width]<5.0627×[writing density]^(−0.5331)[left side]=[line width/acceptance width]=[0.25/3.0]=0.0833[right side]=5.0627×[writing density]^(−0.5331)=0.167

-   -   Therefore, [left side]<[right side] is satisfied.

The reason of setting such conditions in correlation among the linewidth, acceptance width, and the writing density, that is, the approachof embodying the present invention and the basis of the presentinvention are explained below.

As illustrated in FIG. 39, an output characteristic, obtained in themisalignment detection by the conventional optical sensor, is not astraight line at all.

However, as a result of measuring an actual deviation of an alignmentpattern formed on the transfer belt 18 by a digital microscope equippedwith 2,000,000-pixel CCD, the deviation at P7 is substantially zero asillustrated in FIG. 6. More specifically, at P7, the two lines in apattern, in which the line width is set to 24 dots, are perfectlysuperposed on each other.

In the approximate line obtained by plotted spots on the negative sidewith respect to the extreme value, R² is 0.9988, and in the approximateline obtained by plotted spots on the positive side with respect to theextreme value, R² is 0.9996. That is, a result infinitely close to thestraight line (R²=1) is obtained.

As a result of calculating an intersection point by the two lines, apositional deviation is 4.13 micrometers, and actually, the positionaldeviation is a value substantially close to zero. In a pattern in whichthe line width is set to 10 dots, a deviation of the pattern on thetransfer belt 18 is measured in the above manner to obtain the deviationof almost zero.

As explained above, the deviation of the pattern on the transfer beltdetected by the digital microscope is almost zero in the two referencesas the line width. However, when the line width is set to 24 dots, anextremely bad result is obtained such that the positional deviation is34.74 micrometers through calculation based on the output of amisalignment detecting sensor. This deviation is far from the maximumtolerance of the detection error at 600 dpi (=±25.4/resolutiondpi×1000/2 [μm]=±21.2 [μm]). Therefore, the difference between the twopatterns (24-dot pattern and 10-dot pattern) may be caused by ameasurement system.

In order to consider the difference between the two, the relationbetween the light receiving diameter of the sensor and a pattern isillustrated in FIG. 7A to FIG. 7C and FIG. 8A to FIG. 8C.

Patches P1 to P3 as illustrated in FIG. 7A to FIG. 7C are formed with24-dot lines, and patches P1 to P3 as illustrated in FIG. 8A to FIG. 8Care formed with 10-dot lines. The shift amount of the lines in P1 and P2and that in P2 and P3 are formed with 4 dots in order to compare themwith each other on the same basis.

In the experiments, both of the patches with 24-dot lines and thepatches with 10-dot lines make each center of the Bk line coincide withthe center of the light receiving plane.

It is assumed that the output voltage of the diffused light from thesensor has a certain correlation with an increase in the area of a colorline portion in the light receiving plane. Therefore, if the increase inthe area is constant with respect to the difference (in this case, 4dots) between the deviations of the adjacent patches, the output voltageshould be linear.

However, as is obvious from FIG. 7A to FIG. 7C, if the line width iswide as 24 dots, the effect of the “circular” light receiving planebecomes evident. Therefore, if “a variation in the area (P2-1) withrespect to the shift of the color line by 4 dots in P1 and P2” iscompared with “a variation in the area (P3-1) with respect to the shiftof the color line by 4 dots in P2 and P3”, the variation becomes largerin the latter. In other words, the variation is not linear.

This means that if the light receiving plane is circular and the linewidth is wide (which does not mean “acceptance width<line width”) withrespect to a acceptance width (which coincides with the light receivingdiameter of a light receiving sensor, in this case), then the detectionaccuracy is likely to be affected by the shape of the light receivingplane.

When the measurement was conducted by the digital microscope, thecircular shape of the light receiving plane was not taken into account.Then, an enlarged photograph of a patch image observed by the digitalmicroscope was captured into a personal computer in a TIFF format, apart of the patch was cut out to a rectangular area, and an intersectionpoint was calculated by calculating an area ratio between the color lineand the Bk line in the rectangular area. Consequently, the circulareffect (nonlinearity of output) did not become evident.

Photoshop 5.5 produced by Adobe Systems Inc. was used for the softwarefor image processing through measurement by the digital microscope.

Based on the result of the experiment, it is estimated that a certainrelation between the line width of the alignment pattern and theacceptance width of the alignment pattern detecting sensor is requiredto be satisfied in order to ensure required detection accuracy when adeviation is detected with resolution of an arbitrary shift amount orless in an image forming apparatus. More specifically, the image formingapparatus includes an alignment pattern obtained by designating aplurality of lines, as one patch, formed by superposing a line image ofblack as a reference color and a line image of a color other than thereference color, and continuously forming patches in such a manner thata relative positional relation of the two color line images is shiftedby an arbitrary amount. The image forming apparatus also includes analignment pattern detecting sensor (alignment pattern detector) thatdetects the alignment pattern, and a corrector that determines amisalignment between the reference color and the other color and adirection of the misalignment from the output signals of the alignmentpattern detecting sensor, and corrects the misalignment. Further, withregard to the case where a misalignment is detected with a resolution ofan arbitrary shift amount or less, the method of calculating anintersection point using the two approximate expressions has beenexplained as one example of calculation algorithm, but the algorithm isnot limited to this example. It is also possible to detect amisalignment with a resolution of the shift amount or less simply byperforming arithmetic operation through comparison of differentialoutputs between the patches.

In order to derive this relation, it is assumed that “the output of thediffused light of the alignment pattern detecting sensor has a primary,linear relation with respect to the area of the color line image in thelight receiving plane”, and the area of the color line image occupyingthe light receiving plane of the detecting sensor is calculated for eachpatch of a two-color superposed pattern. Then, simulation calculationsuch that the calculated area value is designated as the output value ofeach patch is performed by preparing the error conditions as follows.

A detection error of misalignment is also calculated by calculating anintersection point of two lines, obtained when the area values areplotted corresponding to the “shift amount of a color line image withrespect to the reference color (black) on the x-axis”.

Calculation Expression:

As illustrated in FIG. 9, the area S of a rectangle of one sectionpositioned at a distance “a” from the center of the light receivingplane is expressed by a function of the distance “a” from the center ofthe light receiving plane, such that S=2×[a×tan(acos(a/1.5))]×(25.4/600).

Calculation Condition:

-   -   Pattern construction: a repetition of a Bk line and a color line        is designated as one patch, and a plurality of patches, each in        which the color line is shifted by an arbitrary amount with        respect to the Bk line, are continuously formed.    -   Light receiving diameter of the sensor: diameter=3.0 mm        Error Factor and Reference:    -   Line width (Bk, Color): 3 dots, 6 dots, 12 dots, 18 dots, and 24        dots (calculated by (1 dot=42.3 micrometers (600 dpi)).

One examples of calculation results by the simulation are illustrated inFIGS. 10A, 10B through FIGS. 12A and 12B. The results are obtained basedon the condition such that the line width of Bk and a color is 24 dots(=1.016 millimeters) while the light receiving diameter of the sensor is3 millimeters.

FIG. 10A illustrates the result of the case where the center of the Bkline coincides with the center of the light receiving plane of thesensor. FIG. 10B illustrates the positional relation between the two.FIG. 11A illustrates the result of the case where the center of the Bkline is shifted from the center of the light receiving plane of thesensor by 12 dots. FIG. 11B illustrates the positional relation betweenthe two.

FIG. 12A illustrates the result of the case where the center of the Bkline is shifted from the center of the light receiving plane of thesensor by 36 dots. FIG. 12B illustrates the positional relation betweenthe two. All the positional relations are based on the patch 1 in FIG.2.

The same calculation of the intersection point was also conducted forthe line width: 3 dots, 6 dots, 12 dots, and 18 dots. The results of thecalculations are given to FIG. 13. The x-axis of FIG. 13 indicates“shift quantities of the center of the Bk line with respect to thecenter of the light receiving plane” taken up as the error factor, andthe y-axis indicates the results of calculating the intersection point.

It is obvious from the results that the detection error becomes largeras the line width is increased.

However, the results of FIG. 13 are obtained under the condition thatthe acceptance width (=light receiving diameter) is fixed to 3millimeters. Therefore, as illustrated in FIG. 14, a ratio between theline width and the acceptance width is plotted on the x-axis. Themaximum detection error on the positive side and the maximum detectionerror on the negative side each shown in FIG. 13 are plotted on they-axis.

As explained above, the maximum tolerance of the detection error isdetermined depending on resolution (=writing density). Therefore, asshown in FIG. 15, the resolution is plotted on the x-axis, and the “linewidth-to-acceptance width ratio” of each resolution obtained from themaximum detection errors on the y-axis of FIG. 14 is plotted on they-axis (Maximum tolerance of detection error=±25.4/resolution dpi×1000/2[μm]).

It is obvious from the graph of FIG. 15 that the linewidth-to-acceptance width ratio has to satisfy the relation, as follows,with respect to the writing density (=resolution) of the color imageforming apparatus.[Line width/acceptance width]<5.0627×[writing density(dpi)]^(−0.5331)Based on this relation, because the writing density of the color printerof FIG. 1 is 600 dpi, the following expression is obtained.[Line width/acceptance width]<5.0627×[600(dpi)]^(−0.5331)<0.167

Here, as the acceptance width is 3 millimeters, the line width is asfollows.[Line width]<0.167×3 [mm]<0.501 [mm]<11.8 [dot]

Therefore, the line width has to be set to 11.8 dots or less. Since sucha relation between the line width and the acceptance width exists in arelation with the resolution, the calculation result of a positionaldeviation is 34.74 micrometers, which is the maximum tolerance ofdetection error or more, in the results of experiments (FIG. 39)conducted by setting the line width to 24 dots. Therefore, it ispossible to calculate the positional deviation of 12.91 micrometers,which is the maximum tolerance of detection error or less, in theresults of experiments (FIG. 40) conducted by setting the line width to10 dots that clears the condition of the line width of 11.8 dots or lessthat satisfies the relation.

In actual cases, however, the line width is determined based on themaximum deviation to be corrected of an apparatus. Therefore, a correctorder of determining the line width is as follows. That is, a linewidth-to-acceptance width ratio is obtained using the expression, arequired line width is calculated from the maximum deviation, and aacceptance width is determined based on the line width-to-acceptancewidth ratio.

The method of calculating the required line width from the maximumdeviation to be corrected of the apparatus is explained below.

In the example of FIG. 39, when an intersection point is to becalculated by the two approximate lines, because a deviation of thecentral patch is substantially zero, the output voltage of the patch P6is a minimum value. However, if the deviation in the main scanningdirection is about 12 dots, the output is obtained as illustrated inFIG. 16.

In this case, the intersection point can be calculated using points (P4to P13) covering from the minimum value to the data for the maximumvalues. However, if the deviation is 24 dots that are twice the abovedeviation as illustrated in FIG. 17, the calculation of the intersectionpoint becomes impossible.

Therefore, the line width is desirably twice or more of the maximumdeviation to be corrected of the machine.

In other words, the expression as follows will be satisfied.[Line width]>2×[maximum deviation to be corrected]

When the maximum deviation of the apparatus is 5 dots, the followingexpressions are obtained based on the expression.[Line width]>2×[maximum deviation to be corrected]>2×5 [dot]>10 [dot]

If the line width is calculated based on 1 dot=42.3 micrometers (600dpi), then the line width becomes 423 micrometers or more. The linewidth-to-acceptance width ratio in the case of 600 dpi is as follows.[Line width/acceptance width]<5.0627×[600(dpi)]^(−0.5331)<0.167

Therefore, the acceptance width must be the value as follows.[Acceptance width]>0.423 [mm]/0.167>2.53 millimeters or more

Conversely, by setting so, it is possible to suppress a misalignmenterror to the maximum tolerance or less.

In the examination for far, when the two-color superposed pattern isdetected by the alignment pattern detecting sensor, it has beenlogically studied what basis is effective for setting the line width ofthe alignment pattern and the acceptance width (or light receivingdiameter) with respect to the writing density of the image formingapparatus and the maximum deviation to be corrected of the image formingapparatus.

However, even when the condition is set after the careful study, thedetection error in misalignment becomes sometimes larger as a resultobtained through actually conducted experiment. The result of theexperiment is illustrated in FIG. 18.

The graph in FIG. 18 is obtained by the following manner. That is, analignment pattern is formed with 21 patches on the transfer belt 18.Each of the patches is formed with the Bk line and the color linesuperposed on each other as illustrated in FIG. 2 in such a manner thatthe line width of each color is set to 1000 micrometers and the colorline is shifted by 100 micrometers with respect to the Bk line. The linewidth ratio between cyan (C) and Bk of the respective patches ismeasured by a digital microscope equipped with a 2,000,000-pixel CCD,and is plotted on the y-axis with respect to the “arbitrary shift amountof the color line on the x-axis”.

It is seen that a change rate in the line width ratio with respect tothe “arbitrary shift amount of the color line on the x-axis” decreasesnear the maximal value.

FIG. 19 is a graph illustrating the relation between the shift amount onthe x-axis and the sensor output voltage on the y-axis, when the linewidth of the respective colors is set to 500 micrometers and each patchin which the color line is shifted by 10 micrometers with respect to theBk line is detected by the sensor. From this figure, it is also seenthat the linearity deteriorates near the maximal value and near theminimal value.

These are studied because the output voltage actually measured by thealignment pattern detecting sensor 40 of FIG. 3 indicates such a resultthat the output is saturated near the maximal value. It is confirmedfrom the experiment that the output is saturated in the same manner inthe actual patch, as illustrated in FIG. 18 and FIG. 19.

Therefore, the saturation phenomenon in the output near the maximalvalue is caused not by the sensor, but by the image forming apparatusthat forms the pattern.

As a reason why such a result is obtained, it is confirmed throughobservation by a digital microscope that it is because thickening occursboth in the Bk line and the C line with respect to an ideal line widthof 1000 micrometers.

The cause of such a phenomenon includes influence of the toner densityor the like. This phenomenon becomes noticeable particularly in the caseof using a two-component type developing device in which the line edgeeffect is likely to occur.

In this result, as illustrated in FIG. 20A and FIG. 20B, since both theBk line and the C line are thickened, saturation in the output hasappeared only on the maximal value side. When both of the lines arethickened, even if the C line is shifted from the state of patch A shownin FIG. 20A to the state of patch B shown in FIG. 20B, no change occursin the C line between the Bk lines, and the output becomes the same,which is impossible to detect the shift.

Based on this experimental rule, if the toner density of the Bk line isvery high, and consequently, only the Bk line is thickened, or on thecontrary, if the toner density of the color line is very low and a linehaving a line width faithful to the latent image, it is presumed that asimilar output saturation will occur near the minimal value.

In other words, as illustrated in FIG. 21A and FIG. 21B, even if the Cline is shifted from the state of patch A shown in FIG. 21A to the stateof patch B shown in FIG. 21B, the shift of the C line occurs within therange of the Bk line, and therefore, no change occurs and the outputbecomes the same, which is impossible to detect the shift.

Therefore, as for the data points used for determining the approximateline, it is desired to exclude the maximal value and the minimal valueor the data near the values, in order to eliminate the influence by thepeculiar characteristic to the image forming apparatus as much aspossible. Specifically, only the data of (maximum value+minimumvalue)/2±(maximum value−minimum value)×0.4 is used for calculation, forexample, from the maximum value and the minimum value in the output froma plurality of patches.

In the conventional technology, two approximate lines are obtained bythe data for all the points of continuous patches, and the intersectionpoint is calculated to obtain a deviation. Therefore, the saturationphenomenon in output occurring near the intersection point has not beendiscussed.

Therefore, in the conventional technology, the determination coefficientR² of the two approximate expressions may deteriorate due to thesaturation phenomenon in the output occurring near the extreme values.Consequently, an error may occur in the misalignment obtained by thecalculation of the intersection point.

In this embodiment, a belt made of polyimide having a lightness L*(JISZ8729) of 1.7 is used for the transfer belt 18, on which thealignment pattern Pm is formed. The reason why the configurationincluding this is used is explained in more detail below.

FIG. 22 is a graph plotting the output voltages of the regular reflectedlight of the belt ground portion, the Bk solid patch portion, and Cpatch portion, from the alignment pattern detecting sensor 40 asillustrated in FIG. 3A and FIG. 3B, with respect to the LED current onthe x-axis.

For example, when the output voltage of the respective patch portions atthe time of setting the LED current (=37 mA), in which the outputvoltage of the ground portion of the transfer belt becomes 4.0 volts, isobserved, the result as shown in Table 1 is obtained.

TABLE 1 Output of regular reflected light (IF = 37 mA) Output voltage ofregular reflected light [v] LED current [mA] Belt surface Bk solid Cyansolid 37 4.00 0.12 1.91

When the alignment pattern in FIG. 22 is to be read by the output of theregular reflected light, the area ratio of P1 and P13 is blackline×50%+color portion line×50%, and the area ratio of P7 is blackline×50%+color line×50%. Thus, the sensor output of the respective patchportions becomes substantially as follows.

Case where “Bk line group” is formed over “color line group”:

-   -   The output voltage of P1 and P13=0.12(Bk solid)×0.5+1.91(C        solid)×0.5=1.015 volts.    -   The output voltage of P7=0.12(Bk solid)×0.5+4.0(belt        portion)×0.5=2.06 volts.        Case where “Bk line group” is formed under “color line group”:

Each output voltage of the patches is plotted with respect to “arbitraryshift amount of the color line on the x-axis”, the results are asillustrated in FIG. 23. From FIG. 23, the followings are found. Casewhere detection is performed by regular reflected light:

-   -   (a) The output voltage becomes the largest in the patch (P7)        where the two color lines are perfectly superposed on each        other, and the output voltage of the patch is determined        substantially by the output from the belt ground portion.    -   (b) The output difference between the minimum value (P1, P13)        and the maximum value (P7) decreases when the “black line group”        is over the “color line group”, as compared with when “black        line group” is under the “color line group”.

As explained above, the output of the maximal value (P7), when the patchis detected by the regular reflected light, is determined by the output(∝glossiness) from the belt ground portion. Therefore,

-   -   (c) if glossiness on a portion of the belt decreases due to its        wear with time, a partial defect, or the like, the output of        that portion decreases. In other words, if deterioration of the        belt by wear with time does not allow detection of the output of        that portion, then it is time to replace the belt.

That is, the output voltage of the regular reflected light of the patchportion, to which the belt surface is partially exposed as in P2 to P12shown in FIG. 2, is likely to be affected by noise of the surfaceprofile characteristic expressed by the glossiness of the belt or thesurface roughness Rz. Therefore, for example, if there is a defect inthe ground portion of P6 patch, the output of P6 is different from theoutput of P8 as mirror image with respect to P6. Therefore, the positionof intersection point determined by calculation results in deviationfrom the actual point.

On the other hand, when detection is performed by the diffused light,the positional deviation is detected with being hardly affected by thesurface roughness of the transfer belt 18.

FIG. 24 is a graph plotting the output voltages of the diffused light ofthe belt ground portion, the Bk solid portion, and the C solid portion,from the alignment pattern detecting sensor 40 of FIG. 3A and FIG. 3B,with respect to the LED current on the x-axis.

As illustrated in FIG. 26, the output of the regular reflected light hashigh correlation with the glossiness of an object to be detected (thetransfer belt 18 on which the alignment pattern is formed), whereas theoutput of the diffused light has high correlation with the lightness L*of the object, and has no correlation with the glossiness, asillustrated in FIG. 27. Therefore, the transfer belt 18 having L* of 1.7equipped in the color printer in this embodiment has substantially thesame output characteristic as that of the black toner. As is obviousfrom FIG. 27, the linearity is obtained up to lightness L* of about 40,and the linearity is very high up to lightness L* of 20.

Two cases are considered herein like the previous case, and the resultas shown in Table 2 is obtained for the output voltage in each portion.

TABLE 2 Output of diffused light (IF = 37 mA) Output voltage of diffusedlight [v] LED current [mA] Belt surface Bk solid Cyan solid 37 0.07 0.163.42Case where “Bk line group” is formed over “color line group”:

-   -   The output voltage of P1 and P13=0.16(Bk solid)×0.5+3.42(C        solid)×0.5=1.79 volts.    -   The output voltage of P7=0.16(Bk solid)×0.5+0.07(belt        portion)×0.5=0.115 volt.        Case Where “Bk Line Group” is Formed Under “Color Line Group”:

When the output voltages of the respective patches are plotted withrespect to an “arbitrary shift amount of a color line on x-axis”, theresult as illustrated in FIG. 25 can be obtained. From FIG. 25, thefollowings are found.

Case Where Detection is Performed by Diffused Light:

-   -   (a) The output voltage becomes the smallest in the patch (P7)        where the two color lines are perfectly superposed on each        other, and the output voltage of the patch is determined by the        output voltage of “the color line group”.    -   (b) The output difference between the maximum value (P1, P13)        and the minimum value (P7) is increased when the “black line        group” is over the “color line group”, as compared with when the        “black line group” is under the “color line group”.

As explained above, the output of the maximum value (P7), when detectionis performed by the diffused light, is determined by the output(∝lightness) from the “color line group”. Therefore,

-   -   (c) the output is not affected at all by wear of the belt with        time, its partial defect, or the like.

In other words, since detection performance does not depend on thedeterioration of the belt, the life of the transfer belt can beprolonged.

It is assumed that an alignment pattern is detected by the alignmentpattern detecting sensor. More specifically, the alignment pattern isobtained by designating a plurality of lines, as one patch, in which aline image of black as a reference color and a line image of a colorother than the reference color are superposed on each other, andcontinuously forming patches by shifting a relative positional relationbetween the line images of the two colors by an arbitrary amount. Basedon the assumption, in order to detect the pattern without being affectedby time-varying factors such as wear or a partial defect of the transferbelt, the followings are desirable. That is, (1) detection is performedby the output of the diffused light, (2) the imaging order of the blackline image, as the reference color, is the last in color superpositionon the transfer element, and (3) the lightness (L*) of the transferelement on which the alignment pattern is formed be not larger than 40,preferably not larger than 20.

The result as shown in FIG. 26 is obtained by plotting output values ofregular reflected light when the LED current If is fixed to 20 mA withrespect to glossiness of 60 degrees on the surface of the transfer belton the x-axis, for 42 types of transfer belts having differentglossiness and lightness. The measured values of glossiness shown inthis figure are values obtained by measuring the glossiness, using agloss meter PG-1 manufactured by Nippon Denshoku Industries Co. Ltd.,under the condition of measurement angle of 60 degrees.

The result as shown in FIG. 27 is obtained by plotting outputs ofdiffused light when the LED current is fixed to 20 mA with respect tothe lightness L* on the surface of the transfer belt on the x-axis, forthe same 42 types of transfer belts as illustrated in FIG. 26. Thelightness measured values shown in this figure are values obtained bymeasuring the lightness, using X-Rite 938 manufactured by X-Rite, Inc.,under the conditions of a light source of D50 and a viewing angle of 2degrees.

Results of experiments conducted for confirming the effect of the methodaccording to the present invention are illustrated in FIG. 28. Samplingof output voltages was performed by 500 sampling/sec with respect to thelinear velocity of 125 mm/sec of the image forming apparatus (colorprinter) of the present invention.

Output voltages of the whole patches, slopes of the two lines obtainedfrom the output voltages, y-intercepts, RSQ (=determination coefficient:R²), and the result of calculation of the intersection point are shownin Table 3.

TABLE 3 Output voltage of whole patches and Result of calculation ofintersection point Shift amount P1-P7 P7-P13 P1 −250 1.3782 P2 −2001.0183 P3 −160 0.8042 P4 −120 0.5967 P5 −80 0.4821 P6 −40 0.3110 P7 00.2149 0.2149 P8 40 0.2562 P9 80 0.3659 P10 120 0.5644 P11 160 0.7636P12 200 0.9740 P13 250 1.2329 slope −0.0046 0.0043 y-intercept 0.13190.1069 RSQ 0.9713 0.9703 intersection point 2.83 [μm]

As explained above, the results as follows were obtained in theexperiments. That is, the positional deviation was 2.83 micrometers withrespect to the detection-error maximum value in the positional deviationhaving the light receiving diameter of 3 millimeters and the line widthof 250 micrometers (=about 6-dot line) based on the simulation asillustrated in FIG. 13.

As is clear from the graph of FIG. 28, the outputs of data near theextreme values tend to be saturated.

The result of calculating the intersection point using points excludingthe points of the extreme values and data near the extreme values isshown in Table 4.

TABLE 4 Output voltage excluding extreme value and adjacent data andResult of calculation of intersection point Shift amount P2-P4 P10-P12P2 −200 1.0183 P3 −160 0.8042 P4 −120 0.5967 P10 120 0.5644 P11 1600.7636 P12 200 0.9740 slope −0.0053 0.0051 y-intercept −0.0369 −0.0519RSQ 0.9999 0.9998 intersection 1.44 [μm] point

As explained above, by excluding the data points of the extreme valuesand near the extreme values from the data points used to calculate alinear approximate expression, the determination coefficient R² (=RSQ)of an approximate line representing linearity is largely improved from0.9173 to 0.9999 (P1 to P7) and from 0.9703 to 0.9998 (P7 to P13). As aresult, it is confirmed that high-accuracy detection of the positionaldeviation becomes possible.

In this experiment, the positional deviation of the center patch P7 inthe alignment pattern formed on the transfer belt 18 was observed alsoby the digital microscope equipped with 2,000,000-pixel CCD to confirmthe deviation was zero.

Based on the result of the experiment, by setting the width of linesforming each patch of the alignment pattern so as to satisfy a relationas follows, it is possible to set a misalignment detection error due tonon-linearity of output to the maximum tolerance or less. The relationof[acceptance width]>[line width]/(5.0627×[writingdensity(dpi)]^(−0.5331))is satisfied in a relation between a acceptance width of the alignmentpattern detector and a writing density of an image forming apparatus.More specifically, the image forming apparatus includes an alignmentpattern obtained by designating a plurality of lines, as one patch,formed by superposing a line image of black as a reference color and aline image of a color other than the reference color, and continuouslyforming patches in such a manner that a relative positional relation ofthe two color line images is shifted by an arbitrary amount. The imageforming apparatus also includes an alignment pattern detector thatdetects the alignment pattern, and a corrector that determines amisalignment between the reference color and the other color and adirection of the misalignment, from output signals of the alignmentpattern detector, and corrects the misalignment. Further, the linearityof the two approximate lines used for calculation of the intersectionpoint is improved by excluding the extreme values or data near theextreme values from the data points used for the calculation. As aresult, it is possible to decrease the number of data points, i.e., thenumber of patterns used to calculate the linear approximate expression,up to two points at minimum in each line (up to four patches as awhole).

That is, it becomes possible to largely reduce the processing time formisalignment adjusting operation having nothing to do with the normalprinting operation or making no contribution to productivity.

Furthermore, even if such a sensor (the alignment pattern detectingsensor 40 similar to the conventional sensor) is used, high-accuracydetection of the misalignment becomes possible. Thus, sampling using asampling frequency as low as about 1/100 with respect to theconventional edge detection method is sufficient enough to detect amisalignment.

The misalignment correction based on the alignment pattern detectingsensor 40 and the method is performed by a misalignment correcting unit.The misalignment correcting unit 46 is explained below with reference toFIG. 5.

A light emitting diode 40A is a light emitter of the alignment patterndetecting sensors 40 a, 40 b, and 40 c. The amount of light emitted fromthe light emitting diode 40A is controlled by a light emission amountcontroller 47. A photodiode 40B on an output side is connected to aninput-output (I/O) port 54 via an amplifier 48, a filter 49, ananalog-to-digital (A/D) converter 50, and a First In First Out (FIFO)memory 51.

The detection signals obtained from the alignment pattern detectingsensors 40 a, 40 b, and 40 c are amplified by the amplifier (AMP) 48 topass through the filter 49, and are converted from analog data todigital data by the A/D converter 50.

Sampling of data is controlled by a sampling controller 52, and thesampled data is stored in the FIFO memory 51. The sampling controller 52and a write control board 53 are connected to the I/O port 54.

The I/O port 54, CPU 55, ROM 56, and RAM 57 are connected with eachother by a data bus 58 and an address bus 59.

The ROM 56 stores various programs including a program for calculating amisalignment of the alignment pattern Pm. The program for calculating amisalignment of the alignment pattern Pm includes such conditions thatthe extreme data is excluded from the data points used for theintersection calculation, or the like.

The ROM address, the RAM address, and various input and output equipmentare specified by the address bus 59.

The CPU 55 monitors detection signals from the detecting sensors 40 a,40 b, and 40 c at a predetermined timing. Further, the CPU 55 controlsthe light emission amount of the light emitting diode 40A by the lightemission amount controller 47 so that detection of the alignment patternPm is reliably performed even if the light emitting diode 40A of thedetecting sensors 40 a, 40 b, and 40 c deteriorates, to thereby make theoutput level of the light receiving signal from the photodiode 40Bconstant at all times.

The CPU 55 also performs setting of the write control board 53, based onthe correction amount obtained from the detection result of thealignment pattern (including an alignment pattern for detecting amisalignment in the sub-scanning direction, explained later), in orderto change an angular frequency based on a change of registration in themain scanning direction and registration in the sub-scanning directionand based on a magnification error.

The write control board 53 includes a device that can set the outputfrequency very finely, for example, a clock generator or the like usinga voltage controlled oscillator (VCO), for each color including thereference color. The write control board 53 uses the output of the clockgenerator as an image clock. The CPU 55 also controls a stepping motorfor skew adjustment (not shown) in the optical write unit 16 based onthe correction amount obtained from the detection result of thealignment pattern.

The misalignment correcting unit 46 is comprised of the respectiveelements excluding the alignment pattern detecting sensors 40 a, 40 b,and 40 c. The main controller in the color printer may serve as themisalignment correcting unit 46.

The misalignment adjusting operation by the misalignment correcting unit46 is executed when it coincides with any of the following conditionssuch that (1) power is on, (2) a temperature change in the opticalsystem is not smaller than a predetermined value (for example, 5degrees), and (3) a print job for the certain number of sheets or moreis finished.

In the first embodiment, the alignment pattern for detecting colormisalignment in the main scanning direction is explained. However, whencolor misalignment in the sub-scanning direction (the same direction asan advance direction of the transfer belt 18), an alignment pattern Psas illustrated in FIG. 29 is formed on the transfer belt 18 in the samemode as that of FIG. 4. Such a mode is explained below as the secondembodiment of the present invention.

In the second embodiment, the alignment pattern Ps is obtained in thesame manner as the alignment pattern Pm by designating a plurality oflines, as one patch, in which a line image Bk of block as a referencecolor and a line image C of a color other than the reference color suchas cyan are superposed on each other, and continuously forming patcheseach in such a manner that a relative positional relation between thetwo color line images is shifted by an arbitrary amount.

The alignment pattern may be put in any orientation (angle) with respectto the scanning direction as illustrated in FIG. 30. Furthermore, theshape of the light receiving plane of the alignment pattern detectingsensor 40 may be any of a circle, an oval, and a rectangle.

The present invention is effective on the shape of the light receivingplane of a sensor configured to split light into P wave component and Swave component by using a beam splitter and on the arrangement of thesensor in the pattern scanning direction.

One example of using the beam splitter is explained below with referenceto FIG. 31. An alignment pattern detecting sensor 60 as the alignmentpattern detector of the second embodiment includes one light emittingdiode (LED) 61 as a light emitter, three photodiodes (PD) 62, 63, and 64as light receivers, and two polarized beam splitters (PBS) 65 and 66.

Flood light 67 emitted from the LED 61 is randomly polarized, but thePBS 65 splits the light 67 into a light component (S-wave beam)vibrating vertically with respect to the light incident plane and alight component (P-wave beam) vibrating parallel with respect to thelight incident plane. The S-wave beam 68 is reflected by the PBS 65 andenters the PD 62. The P-wave beam 69 transmits the PBS 65 and is cast tothe alignment pattern on the transfer belt 18.

The polarized state of the P-wave beam 69 reflected by the alignmentpattern becomes random due to irregular reflection, and the P-wave beam69 is split into a P-wave beam 70 and an S-wave beam 71 by the PBS 66.The P-wave beam 70 transmits the PBS 66 and enters the PD 63, and theS-wave beam 71 is reflected by the PBS 66 and enters the PD 64.

In the second embodiment, the PDs 62, 63, and 64 as light receiversreceive diffused reflected component, not the diffused light.

In the first and second embodiments, the examples of applying thepresent invention to the color image forming apparatus of four-drumtandem and direct transfer system are explained. However, as illustratedin FIG. 32, the present invention is applicable to an image formingapparatus of using a method of transferring images to an intermediatetransfer element in the configuration of the four-drum tandem type andcollectively transferring the images to a recording medium in theabove-mentioned manner.

The third embodiment of the present invention that is applied to thistype of color image forming apparatus is explained below.

In this embodiment, the alignment patterns Pm and Ps are formed on anintermediate transfer belt 2 as the intermediate transfer element, andthese patterns are detected by the alignment pattern detecting sensor 40arranged near a support roller 2B. The misalignment correcting unit issimilar to that of the first embodiment.

The configuration and operation of a tandem type color copying machineas the image forming apparatus in this embodiment is schematicallyexplained below. The color copying machine 1 has an image formingsection 1A located at the center of the body of the apparatus, a paperfeed section 1B located below the image forming section 1A, and an imageread section 1C located above the image forming section 1A.

The intermediate transfer belt 2 is arranged as a transfer elementhaving a transfer plane extending horizontally, and a device for formingan image having a complementary relation with a color-separated color isprovided on the upper surface of the intermediate transfer belt 2. Inother words, photosensitive elements 3Y, 3M, 3C, and 3B, as imagecarriers capable of carrying images with toners of colors (yellow,magenta, cyan, and black) having the complementary relation, arearranged in tandem with one another, along the transfer plane of theintermediate transfer belt 2.

The photosensitive element 3 (3Y, 3M, 3C, 3B) is formed of a drumrotatable in the counterclockwise direction. Around each of the drums,devices are provided as follows. That is, a charging device 4 (4Y, 4M,4C, 4B) as a charger that executes the image forming processing in therotation process, an optical write device 5 (5Y, 5M, 5C, 5B) as anexposing unit that forms an electrostatic latent image having apotential V_(L) on each of the respective photosensitive elements 3Y,3M, 3C, and 3B, based on the image information, a developing device 6(6Y, 6M, 6C, 6B) as a developing unit that develops the electrostaticlatent image on the photosensitive element 3 with a toner having thesame polarity as that of the electrostatic latent image, a transfer biasroller 7 (7Y, 7M, 7C, 7B) as a primary transfer unit, a voltage applyingmember 15 (15Y, 15M, 15C, 15B), and a cleaning device 8 (8Y, 8M, 8C,8B). The alphabet added to each reference numeral respectivelycorresponds to the color of the toner, as in the photosensitive element3. The developing devices 6 contain respective color toners.

The intermediate transfer belt 2 is wound around among a plurality ofrollers 2A to 2C, and is capable of moving in the same direction atpositions that face the photosensitive elements 3Y, 3M, 3C, and 3B,respectively. The roller 2C separate from the rollers 2A and 2B forsupporting the transfer plane faces a secondary transfer device 9 withthe intermediate transfer belt 2 put therebetween. The image formingsection 1A in FIG. 32 further includes a cleaning device 10 for theintermediate transfer belt 2.

The surface of the photosensitive element 3Y is uniformly charged by thecharging device 4Y, and an electrostatic latent image is formed on thephotosensitive element 3Y at a writing density (=resolution) of 600 dpibased on the image information from the image read section 1C. Theelectrostatic latent image is visualized as a toner image by thetow-component (carrier and toner) developing device 6Y that contains theyellow toner, and the toner image is attracted to the intermediatetransfer belt 2 by an electric field due to the voltage applied to thetransfer bias roller 7Y and transferred to the intermediate transferbelt 2, as a first transfer step.

The voltage applying member 15Y is provided on the upstream side of thetransfer bias roller 7Y in the rotating direction of the photosensitiveelement 3Y. The voltage applying member 15Y applies a voltage to theintermediate transfer belt 2. Specifically, the voltage has the samepolarity as the charged polarity of the photosensitive element 3Y andhas an absolute value larger than a voltage V_(L) in the solid portion.The voltage is applied so as to prevent toner from being transferredfrom the photosensitive element 3Y to the intermediate transfer belt 2before the toner image comes into an area for image transfer, to therebyprevent toner fly-off due to dust when the toner is transferred from thephotosensitive element 3Y to the intermediate transfer belt 2.

In the other photosensitive elements 3M, 3C, and 3B, the similar imageformation is performed, though the toner color is different, and thetoner images of the respective colors are sequentially transferred tothe intermediate transfer belt 2 so as to be superposed on one another.

The toner remaining on the photosensitive element 3 after the image istransferred is removed by the cleaning device 8, and thereafter, thepotential of the photosensitive element 3 is initialized by a decharginglamp (not shown), for the next image forming step.

The secondary transfer device 9 has a transfer belt 9C wound aroundbetween a charging drive roller 9A and a driven roller 9B and moving inthe same direction as that of the intermediate transfer belt 2. Bycharging the transfer belt 9C by the charging drive roller 9A, amulti-color image superposed on the intermediate transfer belt 2 or asingle-color image carried thereon can be transferred to the paper 28 asa transfer material.

The paper 28 is fed from the paper feed section 1B to a secondarytransfer position. The paper feed section 1B includes a plurality ofpaper feed cassettes 1B1 in which the paper 28 is stacked, a paper feedroller 1B2 that separates and feeds the paper 28 sequentially from theuppermost paper one by one, a conveying roller pair 1B3, and aregistration roller pair 1B4 located on the upstream of the secondarytransfer position.

The paper 28 fed from the paper feed cassette 1B1 is once stopped by theregistration roller pair 1B4, a skew deviation of the paper iscorrected, and the paper is fed to the secondary transfer position bythe registration roller pair 1B4, at a timing at which the front end ofthe toner image on the intermediate transfer belt 2 coincides with apredetermined position at the front end of the paper in the conveyingdirection. A manual feed tray 29 is provided on the right side of theapparatus body so as to be folded. The paper 28 stocked in the manualfeed tray 29 is fed by a paper feed roller 31 toward the registrationroller pair 1B4 through a conveying path joined with a paper conveyingpath from the paper feed cassette 1B1.

The optical write devices 5 control write beams each based on the imageinformation from the image read section 1C or the image informationoutput from a computer (not shown) to emit the write beams correspondingto the image information to the photosensitive elements 3Y, 3M, 3C, and3B, to thereby form electrostatic latent images, respectively.

The image read section 1C has an automatic document feeder 1C1, and ascanner 1C2 having a contact glass 80 as a document placing table. Theautomatic document feeder 1C1 has a configuration such that the documentejected onto the contact glass 80 can be reversed to allow scanning onboth surfaces of the document.

The electrostatic latent image on the photosensitive element 3 isvisualized by the developing device 6 to obtain a visible image, and thevisible image is primarily transferred to the intermediate transfer belt2. The toner image in each color is superposedly transferred to theintermediate transfer belt 2, and then the image is secondarilytransferred collectively to the paper 28 by the secondary transferdevice 9. The paper 28 with the image secondarily transferred thereto issent to a fixing device 11, where an unfixed image is fixed thereon byheat and pressure. The toner remaining on the intermediate transfer belt2 after the secondary transfer is removed by the cleaning device 10.

The paper 28 having passed through the fixing device 11 is selectivelyguided to a conveying path toward a paper output tray 27 and a reverseconveying path RP, by a conveying path switching claw 12 provided on thedownstream side of the fixing device 11. When conveyed toward the paperoutput tray 27, the paper 28 is ejected onto the paper output tray 27 bya paper ejecting roller pair 32, and stacked. When guided to the reverseconveying path RP, the paper 28 is reversed by a reversing device 38,and then sent to the registration roller pair 1B4 again.

With the configuration, in the color copying machine 1, by scanning adocument placed on the contact glass 80 through exposure or by the imageinformation from a computer, an electrostatic latent image is formed onthe uniformly charged photosensitive element 3. After the electrostaticlatent image is visualized by the developing device 6 to obtain a tonerimage, the toner image is primarily transferred to the intermediatetransfer belt 2.

The toner image on the intermediate transfer belt 2 is directlytransferred to the paper 28 sent out from the paper feed section 1B,when it is a single image. If it is a multi-color image, the tonerimages are superposed by repeating the primary transfer, and theseimages are secondarily transferred to the paper 28 collectively.

The paper 28 with the images secondarily transferred thereto is conveyedto the fixing device 11 where an unfixed image is fixed, and the paper28 is ejected onto the paper output tray 27, or reversed and sent to theregistration roller pair 1B4 again for forming images on both surfacesof the paper.

The fourth embodiment of the present invention will be explained below.This invention can be similarly executed in a color image formingapparatus of a type in which toner images in the respective colors areformed, using one photosensitive drum and a revolver type developingdevice, the respective toner images are transferred to an intermediatetransfer element by superposition, and the toner images are collectivelytransferred to the recording medium as a sheet-type recording medium.One example of the embodiment is shown in FIG. 33.

In the fourth embodiment, the alignment patterns Pm and Ps are formed onan intermediate transfer belt 426 as the intermediate transfer element,and these patterns are detected by the alignment pattern detectingsensor 40 arranged near a drive roller 444. The misalignment correctingunit is the same as that of the first embodiment.

The configuration of the color copying machine as the image formingapparatus in this embodiment is explained below.

In the color copying machine, an optical write unit 400 as an exposingunit converts the data for a color image from a color scanner 200 tooptical signals and performs optical writing corresponding to thedocument image, to thereby form an electrostatic latent image on aphotosensitive drum 402 as an image carrier at a writing density(=resolution) of 600 dpi.

The optical write unit 400 includes a laser diode 404, a polygon mirror406, a rotation motor 408 for the mirror 406, an f/θ lens 410, and areflection mirror 412.

The photosensitive drum 402 is rotated in the counterclockwise directionas shown by an arrow. Around the photosensitive drum 402, there arearranged a photosensitive element cleaning unit 414, a decharging lamp416, a potential sensor 420, a developing unit selected from a revolvertype developing device 422, a developing density pattern detector 424,and the intermediate transfer belt 426 as the intermediate transferelement.

The revolver type developing device 422 has a developing unit for black428, a developing unit for cyan 430, a developing unit for magenta 432,a developing unit for yellow 434, and a rotation drive section (notshown) that rotates the respective developing units. The developingunits are so-called two-component developer type developing units eachcontaining mixed developer of carrier and toner. Each of the developingunits has the same configuration as the developing device 4 shown in theabove embodiment. The same goes for the conditions and specification forthe magnetic carrier.

In the standby state, the developing device 422 is set at the positionfor black development. When the copying operation is started, the datafor a black image is started to be read in the color scanner 200 at apredetermined timing, and optical writing with the laser beam is startedbased on the image data, and an electrostatic latent image (black latentimage) is formed.

In order to start developing from the front end of the black latentimage, a developing sleeve is rotated before the front end of the latentimage reaches the development position of the developing unit for black428, to develop the black latent image with the black toner. A tonerimage having a negative polarity is formed on the photosensitive drum402.

Thereafter, the developing operation in the area of the black latentimage is continued, but when the rear end of the latent image has passedthe black developing position, the revolver type developing device 422is promptly rotated from the developing position for the black to thedeveloping position for the next color. This operation is completed atleast before arrival of the front end of a latent image that is formedbased on the next image data.

When the image forming cycle is started, at first, the photosensitivedrum 402 is rotated in the counterclockwise direction as shown by thearrow, and the intermediate transfer belt 426 is rotated in theclockwise direction, by a drive motor (not shown). With the rotation ofthe intermediate transfer belt 426, a black toner image is formed, acyan toner image is formed, a magenta toner image is formed, and ayellow toner image is formed, and finally, the toner images aresuperposed on each other on the intermediate transfer belt 426 (primarytransfer), in order of black (Bk), cyan (C), magenta (M), and yellow (Y)to thereby form a toner image.

The intermediate transfer belt 426 is stretched between respectivesupport members, that is, a primary transfer electrode roller 450 facingthe photosensitive drum 402, the drive roller 444, a secondary transferfacing roller 446 facing the secondary transfer roller 454, and acleaning facing roller 448A facing the cleaning unit 452 that cleans thesurface of the intermediate transfer belt 426. The intermediate transferbelt 426 is controlled by the drive motor (not shown).

The respective toner images of black, cyan, magenta, and yellowsequentially formed on the photosensitive drum 402 are accuratelyregistered in order on the intermediate transfer belt 426 to therebyform an image with the four colors superposed on one another on thetransfer belt 426. This image is collectively transferred to the paperby the secondary transfer facing roller 446.

A feed paper bank 456 has recording paper cassettes 458, 460, and 462that store paper in various sizes different from the size of the paperstored in the cassette 464 of the apparatus body. Of these cassettes,size-specified paper is fed from a corresponding cassette by a paperfeed roller 466 and conveyed toward the registration roller pair 470. InFIG. 33, reference numeral 468 denotes a manual feed tray for paper foran over head projector (OHP), thick paper, and the like.

When the image formation is initiated, the paper is fed from a feedingport of any of the cassettes, and stands by at a nip portion of theregistration roller pair 470. The registration roller pair 470 is drivenso that when the front end of the toner image on the intermediatetransfer belt 426 approaches the secondary transfer facing roller 446,the front end of the paper coincides with the front end of the image, tothereby perform registration between the paper and the image.

In this manner, the paper is superposed on the intermediate transferbelt 426, and passes under the secondary transfer facing roller 446 towhich a voltage having the same polarity as that of the toner isapplied. At the time of the application, the toner image is transferredto the paper. Subsequently, the paper is decharged, and separated fromthe intermediate transfer belt 426 to be moved onto the paper conveyorbelt 472.

The paper with the four-color superposed toner image is conveyed to thebelt fixing type fixing device 470 by the paper conveyor belt 472, andthe toner image is fixed on the paper by the fixing device 470 usingheat and pressure. The paper with the image fixed is ejected to theoutside of the apparatus by an ejection roller pair 480, and stacked ina tray (not shown). Through the steps, a full color copy is obtained.

The alignment pattern detecting sensor explained above in the presentinvention is applicable to any image forming apparatus that detects amisalignment and corrects it, by detecting an alignment pattern formedwith a plurality of two-color superposed patches. Therefore, this sensoris also applicable as a misalignment detecting sensor of an ink jetdevice. One example of this embodiment is explained with reference toFIG. 34 to FIG. 36.

In the fifth embodiment, the schematic configuration and printingfunction of an ink jet printer 500 as an image forming apparatus areexplained with reference to FIG. 34. The ink jet printer 500 has aprinting mechanism section 502 inside the body 501 of the printer. Thissection 502 includes a carriage movable in the main scanning direction,a recording head comprising an ink jet head mounted on the carriage, andan ink cartridge for supplying ink to the recording head,

A paper feed cassette 504 that can accommodate paper 503 as a sheet-typerecording medium is provided in the lower part of the printer body 501,and the paper feed cassette 504 is detachably provided with respect tothe printer body 501 from the front side (from left side in the figure)of the printer.

A manual feed tray 505 is provided on the front face of the printer body501 so as to be freely opened or closed. Therefore, the paper 503 fedfrom the paper feed cassette 504 or the manual feed tray 505 is conveyedto the printing mechanism section 502 where a specified image is printedthereon at a writing density (=resolution) of 600 dpi, and the paper 503is ejected to a paper output tray 506 provided at the backside of theprinter body 501. An upper cover 507 is provided on the upper surface ofthe printer body 501 so as to be freely opened or closed.

In the printing mechanism section 502, the carriage 510 is slidably heldby a main guide rod 508 and a sub-guide rod 509 supported between rightand left side plates (not shown) in the main scanning direction(perpendicular to the face of paper). A recording head 511 includes anink jet head having nozzles for discharging ink drops for the respectivecolors of yellow (Y), cyan (C), magenta (M), and black (Bk), and isprovided on the lower side of the carriage 510. Respective inkcartridges 512 for supplying the ink of the respective colors to therecording head 511 are provided on the upper side of the carriage 510 soas to be replaceable.

The recording head 511 may be one in which a plurality of heads fordischarging ink drops in each color are arranged along the main scanningdirection, or one in which one head having nozzles for discharging inkdrops in each color is used.

Provided under the recording head 511 is a conveyor belt 515 thatelectrostatically attracts and conveys the paper 503 is wound aroundbetween a conveying roller 513 and a driven roller 514, in order toconvey the paper 503 in the sub-scanning direction with respect to theposition of printing by the recording head 511. Tension is given to theconveyor belt 515 by an intermediate roller 516.

At a position facing the conveying roller 513, with the conveyor belt515 put therebetween, a bias roller 517 for charging the conveyor belt515 is disposed. A press roller 518 for pressing the paper 503 againstthe conveyor belt 515 is disposed near a starting point of a flatportion of the conveyor belt 515. Here, the starting point of the flatportion means a portion where the conveyor belt 515 is separated fromthe conveying roller 513 to be stretched in parallel with the recordinghead 511.

The paper 503 stored in the paper feed cassette 504 is separated one byone in order of the uppermost paper by a paper feed roller 519 and afriction pad 520, and conveyed by a curved guide member 521 toward a nipportion of the bias roller 517 and the conveyor belt 515.

The alignment pattern detecting sensor 40 that detects the alignmentpattern Pm formed on the paper 503 is provided above the conveyor belt515 on the downstream side in the paper conveying direction. Themisalignment correcting unit is the same as that of the firstembodiment.

FIG. 35 is an enlarged diagram illustrating the periphery of theconveyor belt 515. A plate-like retainer 522 may be provided instead ofthe press roller 518 shown in FIG. 34.

As illustrated in FIG. 36, an alignment pattern Pk as illustrated inFIG. 37 is formed on the paper 503 as a transfer element on which analignment pattern is formed, and this pattern is detected by thealignment pattern detecting sensor 40 to thereby correct the colormisalignment.

As explained above, according to one aspect of the present invention, itis possible to improve the linearity, that is, to improve detectionaccuracy of the alignment pattern detecting sensor or the alignmentpattern detector of the image forming apparatus.

Moreover, the alignment pattern detecting sensor or the alignmentpattern detector of the image forming apparatus obtains the followingadvantageous effects.

-   -   (1) It is possible to perform highly accurate detection at a        sampling frequency as low as about 1/100 as compared with the        conventional edge detection method.    -   (2) As a result of (1), there is no need to speed up processing        of the processing section after the sampling process. Thus, it        is possible to largely reduce the cost of the electronic        hardware.    -   (3) Since the linearity of the two lines is extremely improved,        it is possible to largely reduce the number of patches that        forms the alignment pattern.    -   (4) As a result of (3), the processing time required for        adjustment like misalignment adjustment that has nothing to do        with ordinary printing can be largely reduced. Thus, it is        possible to extremely improve productivity.

Furthermore, it is possible to enhance the accuracy of misalignmentdetection in the alignment pattern detecting sensor or the alignmentpattern detector of the image forming apparatus.

Moreover, it is possible to prevent calculation of an intersection pointfrom being disabled.

Although the invention has been described with respect to a specificembodiment for a complete and clear disclosure, the appended claims arenot to be thus limited but are to be construed as embodying allmodifications and alternative constructions that may occur to oneskilled in the art which fairly fall within the basic teaching hereinset forth.

1. An image forming apparatus comprising: an alignment pattern formingunit that forms an alignment pattern on a medium as a plurality ofpatches shifted with respect to one another by an arbitrary amount, eachof the patches being formed by superposing a line image of a referencecolor and a line image of a sample color other than the reference color;an alignment pattern detector that detects the alignment pattern; and amisalignment correcting unit that determines, based on output signals ofthe alignment pattern detector, an amount and a direction of amisalignment between the line images of the reference color and samplecolor, and corrects the misalignment, wherein an acceptance width of thealignment pattern detector, a line width of the alignment pattern, and awriting density of the image forming apparatus satisfy followinginequality(line width)/(acceptance width) <(α×writing density (dpi))^(−β).
 2. Theimage forming apparatus according to claim 1, wherein α is 5.0627, and βis 0.5331.
 3. The image forming apparatus according to claim 1, whereinthe alignment pattern is formed on the medium by superposing a lineimage of a reference color and a line image of a sample color other thanthe reference color to make a plurality of lines as one patch, andarranging a plurality of patches in which a relative position betweenthe line images of the two colors is continuously shifted by apredetermined amount.
 4. The image forming apparatus according to claim1, wherein the reference color is black.
 5. The image forming apparatusaccording to claim 1, further comprising: a plurality of image carrierson which toner images are formed; and a transfer element to which thetoner images are sequentially transferred, wherein the reference coloris set to black, and a toner image of the black is transferred lastly tobe superposed on other toner images or the transfer element.
 6. Theimage forming apparatus according to claim 5, wherein a lightness L* ofthe transfer element, on which the alignment pattern is formed, is equalto or less than
 40. 7. An image forming apparatus comprising: analignment pattern forming unit that forms an alignment pattern on amedium as a plurality of patches shifted with respect to one another byan arbitrary amount, each of the patches being formed by superposing aline image of a reference color and a line image of a sample color otherthan the reference color; an alignment pattern detector that detects thealignment pattern; and a misalignment correcting unit that determines,based on output signals of the alignment pattern detector, an amount anda direction of a misalignment between the line images of the referencecolor and sample color, and corrects the misalignment, wherein anacceptance width of the alignment pattern detector, a line width of thealignment pattern, and a writing density of the image forming apparatussatisfy following inequality(line width)/(acceptance width) <(α×writing density (dpi))^(−β), whereinthe acceptance width is determined based on a required line widthsatisfying the inequality, wherein the required line width is calculatedfrom a maximum misalignment between the line images of the referencecolor and sample color.
 8. The image forming apparatus according toclaim 7, wherein the required line width is equal to or more than twicethe maximum misalignment.